The position of an electrical charge within an electric field represents a potential energy that may be converted to work through displacement of the charge. Stable separation of electrical charge gives rise to storage of energy as potential energy, and is typically accomplished chemically, in batteries, or electrostatically, in capacitors. Electrostatic storage in capacitors is limited by dielectric breakdown, while chemical batteries suffer from the drawback of limited charge/discharge rates. Reverse-biased diodes used for energy storage are limited by field emission, avalanche breakdown, and Zener breakdown. Limitations of existing technologies are also discussed in the following pages.
Electrical energy from a DC power source can be stored in conventional capacitors, electrochemical capacitors, chemical batteries, and diodes. Conventional parallel plate capacitors can be charged and discharged quickly and they have a virtually unlimited life time, but their energy density is small, because of dielectric breakdown. For instance, when the electric field E exceeds about 0.118 V/nm in a 25-μm thick Teflon sheet coated with w=100 nm thin-film electrodes, a spark discharges the capacitor and the energy is lost as heat. This limits the energy density in aluminum-Teflon-aluminum capacitors to about
      u    =                                        ε            0                    ⁢                      ε            r                    ⁢                      E            2                          2            =              265        ⁢                                  ⁢        kJ        ⁢                  /                ⁢                              m            3                    ⁡                      (                          125              ⁢                                                          ⁢              J              ⁢                              /                            ⁢              kg                        )                                ,where ∈0=8.85×10−12 F/m is the vacuum permittivity and ∈r=2.15 is the relative permittivity of Teflon. Commercial capacitors have energy densities up to 300 J/kg. The inductance of the capacitor circuit limits the rate at which the capacitor can be charged and discharged to about 107 W/kg.
Solid state diodes, such as varactor diodes, can be used for energy storage as well. Semiconductor diodes with reverse bias store energy in the depletion layer. However, field emission, avalanche breakdown and Zener breakdown limit the electric field to about E=0.02 V/mm (in silicon with donor concentration N=1014 cm−3), and the energy density is less than in conventional capacitors. Field emission, i.e. quantum mechanical tunneling of carriers through the band gap, is the dominant breakdown mechanism for highly doped p-n junctions. Zener breakdown occurs when the electric field becomes large enough to excite valence electrons in the depletion zone directly into the conduction band. Avalanche breakdown occurs when the minority carriers are accelerated in the electric field in the depletion region to sufficient energies that they can excite valence electrons through collisions. Energy storage in semiconductor junctions is further limited by the fact that the depletion zone is not a perfect insulator and reverse saturation currents discharge the diode. The charge-discharge rate of diodes, limited by the mobility of the carriers, is much higher than in batteries. Tunnel junction have the highest switching speeds (up to 5 GHz), but their reverse-biased breakdown voltage is small.